Keying circuit



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KEYING CIRCUIT Filed April 26, 1941 Fig.1

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Patented May 19, 1942 KEYING CIRCUIT Victor James Cooper, Writtle, England, assignor to Radio Corporation oi America, a corporation of Delaware Application April 26,1941, Serial No. 390,505 In Great Britain April 3, 1940 2 Claims.

This invention relates to keying arrangements for radio and like systems and provides what may be referred to as softening whereby to prevent key-clicks.

A complete description of the invention fol- 5 lows, wherein reference is made to the accompanying drawing showing,

Figure 1 which represents a circuit arrangement for shaping the voltage wave form suitably to avoid key-clicks; and

Fig. 2 shows graphically various wave form representations.

Softening by the usual CR circuits produces a voltage wave form such as is illustrated in Fig. 2 oi the accompanying drawings, at a. This wave form exhibits an exponential rise and fall of voltage'on make and break respectively for mark and space, and it will be seen that there exists a very high rate of change at the point X at which pointthe level of the keyed signal is high. This tends to produce an undesirable click which can be reduced or minimized only by reducing the initial slope of the curve X, Y. Such reduction has the disadvantage, however, that it produces a long tail" 2 which either reduces intelligibility or speed, since dots tend to run together.

According to the invention a keying circuit arrangement for radio or like transmitters includes a valve having at least a cathode, a control grid, and an anode: an input network connected between said control grid and cathode, said input network including a resistance-shunted capacity; a keying circuit including keying contacts. a series impedance and a source of potential connected across said input network, and an output circuit connected between said anode and cathode, said output circuit including a source of anode potential and an impedance.

Preferably there is included in the cathode leg of said valve a bias resistance and, if desired, this reactance may be capacity shunted.

Referring now to Fig. 1, a circuit arrangement is illustrated including a variable-mu discharge tube I, such as that known commercially as an 4 RCA type 35 tube. Between the source SI and the cathode of this tube is connected a self-bias resistance RI, and in shunt therewith an ad- 'I'he control grid is connected to a resistance-shunted capacity network 60 R2, C2 and also through resistance R3 to a keying circuit 2 which includes a source of keying potential S2. The negative terminal of source SI is also connected to R2 and C2. The anode and screen grid circuits are connected to the positive terminal of source SI. The keying source S2, if independent of the source SI (as shown), has its positive terminal connected to the cathode of the tube I. The anode circuit includes a resistance R and from across this last mentioned resistance (or part thereof) amplified keying potentials may be derived at terminals 3 for impress upon the carrier energy of a radio transmitter. Across the source of anode potential SI there is connected a potential-dividing resistance R5 'from which positive potential for the screening grid G2 of the valve is taken at point P.

With this arrangement softening of the beginning and end of the keying output impulses from the valve is eflfected without, as is the case'with the usual CR circuits, the disadvantage of producing a high rate of change of the slope of the wave form at the point where the carrier, at a maximum, is cut oil. Wave forms for the input and output voltages 2 and 3 are shown at b and c in Fig. 2.

If desired, a condenser CI may be connected across the cathode-leg bias resistance RI.

Circuit values are so chosen that, when the keying contacts are closed, the valve is biased to cut-oil, and no difference of potential appears across the terminals of the output resistance R4. In this condition a potential diflference exists across the resistance-shunted capacity input network R2, C2. Furthermore, when the valve is biased to cut-oil, and when the screen grid current is negligible in relation to the current through R5, then there will be substantially no potential difi'erence across the cathode-leg bias resistance RI. Thus, the screen grid G2 can be -maintained at a substantially constant potential determined by the potential of the source SI and the position of the tapping point P on' the potential divider R5. When the keying contacts are opened the condenser 02 in the input network will discharge through the resistance R2 in that network, and the till then existing negative potential componenton the control grid GI due to the source of keying potential will decrease in accordance with an exponential law. This has the eiiect of permitting anode current to flow through the valve I and a gradually increasing potential component will appear across the cathode leg bias-resistance RI. This latter potential component will tend to delay the growth of current in the valve, and these two effects produce the desired "softening" in the rise of anode current.

If a condenser CI is connected across the oathode-leg bias-resistance RI, when the voltage is building up across this resistance the shunting condenser will charge up at a rate dependent on the voltage across the resistance RI, the rate of change of current in the anodecircuit, and the value of the capacity CI. If the condenser is large, until a steady state is reached, the selfbias will be less than the bias would be if the condenser were absent. This condenser thus provides, after the softening following the opening of the keying contacts, a transitory state during which the anode current is in excess of the steady state anode current and thereby a momentary peak of excess voltage which corresponds to a curbing voltage which can be utilized to improve the intelligibility of high speed keying, firstly, by reducing the tail" of the break curve to zero and, secondly, by improving the ratio of mark time to space time. The wave form for this condition is shown at din Fig. 2.

When the keying contacts are again closed, the condenser C2 in the input network will again charge according to an exponential law, so that the negative grid potential will increase accordingly, and the anode current fall. The fall of anode current will be opposed by the potential across the self-bias resistance RI so that a delay, with consequent softening of the make of the output impulse, will be effected.

Suitable values for the various components are as follows: keying voltage 100 volts, keying resistance 80,000 ohms, condenser in the input network V2 microfarad, resistance in that network 55,000 ohms, cathode-leg resistance 5,000 ohms, anode potential 215 volts and screening grid potential 100 volts (all potentials being referred to the potential of the cathode) and anode resistance 25,000 ohms.

A further degree of "softening" can be achieved if desired by making R5 9. high resistance potentiometer or by omitting the part of the resistance R5 connected between the tapping point P and the negative terminal of the source of supply SI so that the screening grid S2 of the valve is maintained at a potential dependent upon the current taken by the screening grid.

I claim:

1. In a keying circuit for a radio transmitter, an electron discharge tube having at least a cathode, an anode, a control grid and a screen grid, keying means connected through an adjustable resistor to the control grid, also through a source of keying potential equivalent to a cut-oi! bias, and through a second resistor to said cathode, an adjustable capacitor in shunt with said second resistor, an output circuit which includes said second resistor, a source of anode potential, and a load impedance, a voltage divider connected across the terminals of said anode potential source, a connection from a suitable point on said voltage divider to said screen grid, and a capacity shunted grid leak resistor connected between said control grid and the terminal of said second resistor remote from the cathode.

2. The combination according to claim 1 and further characterized in that the parameters of each said resistor, each said capacitor, and the said potential sources are so determined as to produce keying impulses in the output circuit which possess a substantially round cornered and flat-topped wave formation.

VICTOR JAMES COOPER. 

